Re-entry vs fireball

Robert H. McNaught (
Wed, 11 Mar 1998 16:34:32 +1100 (EST)

The question of satellite vs meteor is raised regularly.  Unfortunately
there is no single criterion that can differentiate between these possibilities
based on a single visual observation.  The answer is to be found somewhere
in the following

a) Is the path consistent with a predicted satellite re-entry?

b) Is the duration longer or shorter than, say, 15 seconds?

c) Is it brighter than mag -8?

d) Does the object have many fragments?

e) What is the direction of motion?

A satellite re-entry will typically last many tens of seconds, whereas a
meteoric fireball would rarely last that long.  Satellite usually have a
progressive breakup during re-entry and the fragments trail in the same line
as each other.  It is certainly possible for a meteoric fireball to enter
the atmosphere at a shallow angle, but it would have to be massive to have
a duration longer than 15 seconds, and the brightness would therefore
usually be greater than, say, mag -8.  Such objects often show a
progressive break-up but could probably be separated by the brightness (e.g.
Peekskill fireball, 40 sec durn, lots of fragments, mag brighter than -13).
Satellite re-entries are rarely, if ever, brighter than mag -8.

A single object of short (few seconds) duration will never be a satellite
re-entry and it would be rare for a re-entry to follow an east to west path.

Of course, determining the trajectory of the object will give a strong clue
as to the nature.  Satellite re-entries will typically have a VERY shallow
trajectory (few degrees).  A steep trajectory (3-D, not the 2-D apparent
trajectory as seen from certain directions) will invariably be a fireball.
Finally, if the velocity can be calculated, a satellite re-entry will be
below 8km/sec, whereas a meteoric fireball above 11 km/sec, often well above.

It should be noted that meteoric fireballs of mag -4 or brighter are some
hundreds of times more frequent than satellite re-entries.  Perhaps this
realisation would make the NEO threat more real to people.  An object has
to have a very long duration to have a reasonable chance of being a re-entry.

Hope this helps!

Cheers, Rob McNaught